1. Field of the Invention
The present invention relates to an Organic Light Emitting Display (OLED) and its method of fabrication, and more particularly, the present invention relates to an OLED and its method of fabrication using fewer masks.
2. Description of the Related Art
In general, a Thin Film Transistor (TFT) includes a semiconductor layer including a gate electrode, source and drain regions formed on the gate electrode, a channel region disposed between the source and drain regions; and source and drain electrodes that come in contact with the source and drain regions of the semiconductor layer.
The semiconductor layer of the TFT configured as described above is generally formed of amorphous silicon or poly-silicon. If the semiconductor layer is formed of amorphous silicon, mobility is low so that it is difficult for a driving circuit to operate at a high speed. On the other hand, if the semiconductor layer is formed of poly-silicon, mobility is high whereas a threshold voltage is non-uniform. Therefore, a separate compensating circuit must be added.
In a conventional method of fabricating a TFT using Low Temperature Poly-Silicon (LTPS), a high-cost process, such as a laser heating treatment, is needed, and it is not easy to control characteristics. Therefore, it is difficult to apply the conventional method of fabricating a TFT using LTPS to a large-area substrate.
In order to solve these problems, studies for using an oxide semiconductor as a semiconductor layer have been recently conducted.
Japanese Patent Publication No. 2004-273614 teaches a TFT using an oxide semiconductor, which is made of zinc oxide (ZnO) or includes ZnO as a main element, as a semiconductor layer.
However, if such a TFT is applied to an OLED, the following processes are added. In order to planarize a substrate on which a TFT is disposed, a planarization layer must be formed on the entire surface of the substrate on which the TFT is disposed. Furthermore, in order to electrically connect the TFT to the OLED, a contact hole is be formed on the planarization layer. Therefore, processes of fabricating an OLED may be complicated, and the number of processes using a mask may be increased.